Autophagy is a process whereby cells convert proteins and organelles into amino acids as a source of food. Many cells in the human body rely on autophagy to maintain homeostasis, especially when insulin levels are low. Autophagocytosis may play a role in human disease and aging. In eukayrotic cells autophagy occur constitutively at low levels in all cells to perform housekeeping functions such as destruction of dysfunctional organelles. Dramatic upregulation occurs (e.g., cytoplasmic and organelle turnover) in the presence of external stressors (starvation, hormonal imbalance, oxidation, extreme temperature, and infection), and internal needs (generation of source materials for architectural remodeling, removal of protein aggregates). Autophagy is highly regulated through the coordinated action of various kinases, phosphatases, and guanosine triphosphatases (GTPases).
At least three different autophagy mechanisms are known, all of which result in targeting of cytosolic proteins and organelles to the lysosome in order to provide amino acids and energy in the form of catabolites. These types are macroautophagy, microautophagy, and chaperone-mediated autophagy.
Macroautophagy is a major inducible pathway for the general turnover of cytoplasmic constituents in eukaryotic cells and also plays a significant role in the degradation of active cytoplasmic enzymes and organelles during nutrient starvation. Macroautophagy involves the formation of double-membrane bound autophagosomes which enclose the cytoplasmic constituent targeted for degradation in a membrane bound structure, which then fuse with the lysosome (or vacuole) releasing a single-membrane bound autophagic bodies which are then degraded within the lysosome (or vacuole). MAP1A and MAP1B are microtubule-associated proteins which mediate the physical interactions between microtubules and components of the cytoskeleton. These proteins are involved in formation of autophagosomal vacuoles (autophagosomes). MAP1A and MAP1B each consist of a heavy chain subunit and multiple light chain subunits. Apg8a is one of the light chain subunits and can associate with either MAP1A or MAP1B. The precursor molecule is cleaved by APG4B/ATG4B to form the cytosolic form, Apg8a-I. This is activated by APG7L/ATG7, transferred to ATG3 and conjugated to phospholipid to form the membrane-bound form, Apg8a-II.
Microautophagy circumvents the autophagosomic step of macrophagy, and begins with the direct uptake of cytosolic material via invaginations and pinching off of the lysosomal membrane. The internalized cytosolic components are digested by lysosomal enzymes released when the vacuolar membrane disintegrates, as in macroautophagy.
In chaperone-mediated autophagy, specific chaperone proteins bind to target proteins containing a KFERQ (SEQ ID NO: 1) sequence and channel these proteins to the surface of the lysosome. These proteins bind to Lamp2a and are then transported across the lysosomal membrane with the assistance of lysosomal chaperones, after which they are degraded by vacuolar proteases.
Mizushima, et al., describes autophagy as promoting both cell survival and cell death. By maintaining homeostasis during times of cellular stress, autophagy generally promotes survival when it is controlled. See Mizushima, N., et al., “Autophagy fights disease through cellular self-digestion” Nature (2008) 451:1069-1075.
However, dramatic upregulation of autophagy via Beclin 1 overexpression brings about cell death. Mizushima, et al., describe how the autophagy and apoptosis pathways share many common regulatory factors, with the likelihood of significant cross-talk between these pathways in the cell. Since apoptosis is known to be implicated in human disease, autophagy also is likely an important phenomenon to target in order to treat disease.
Proteins that regulate autophagy in cancer cells make attractive therapeutic and diagnostic targets. Cancer cells rely on autophagy in order to evade anti-cancer treatments designed to reduce nutrient supply and enhance the stress on rapidly dividing cells. A compound that downregulates autophagy may be a useful additional drug in cancer treatment. Mizushima, et al., state that since autophagy may help prevent cancer, there is a potential need to target autophagy in a context-specific manner. The targeting of specific autophagy regulatory proteins rather than a targeting of autophagy in general may be critical in developing a treatment of cancer as well as new modes of diagnosing cancer.
Alterations in the autophagy degradation pathway have been described in normal brain aging and in age-related neurodegenerative diseases including Alzheimer's and Parkinson's diseases. See Nixon, R., “Autophagy in neurodegenerative disease: friend, foe, or turncoat?” Trends in Neurosciences (2006) 29(9):528-535. An improper clearance of proteins in these diseases may result either from a compromise in the autophagy degradation pathway or induced alterations in this pathway, and may result in neuron dysfunction and neuron loss. The targeting of specific autophagy regulatory proteins, rather than a targeting of autophagy in general, may be critical in developing a treatment of neurodegenerative diseases as well as new modes of diagnosing neurodegenerative diseases. Therefore, there exists a need to develop an assay to monitor the activity of autophagy proteins that does not rely exclusively on protein localization.